OBIGIN OF INTESTINAL WOEMS. 13 



y 



are, and remain, dissimilar to their parent, but bring forth a new 

 generation, whose members either themselves, or in their descend- 

 ants, return to the original form of the parent animal." 



Any one who has not familiarized himself with the fun- 

 damental idea of this doctrine of the alternation of generations, 

 may easily imagine it to be nothing but a modification of the 

 long well-known metamorphosis, exemplified by the tadpoles of 

 frogs and toads, or by the larvae and chrysalises of most insects. 

 This is, however, by no means the case. Those reptiles and 

 insects that are subject to metamorphosis, no doubt bring forth 

 young that differ from the parent, but there are two respects in 

 which the act of simple metamorphosis widely differs from the 

 highly complex alternation of generations. 



Although Steenstrup has already particularly noticed these 

 two grounds of difference in his definition of the alternation of 

 generations, I deem it not altogether superfluous on my part 

 once more to draw especial attention to these important diver- 

 gences, if only for the benefit of those who are unfamiliar with 

 the phenomena. The first point of difference between the 

 alternation of generations, and metamorphosis, is, that the young 

 of those animals whose mode of development comes under the 

 former head, are not only unlike their parent at first, but remain 

 so : the second distinction rests on the important fact that this 

 young generation, so dissimilar to the parent animal, brings 

 forth new creatures, which either themselves, or in their descend- 

 ants, revert to the original form of the first parent. Whereas, 

 on the other hand, in simple metamorphosis, the dissimilar 

 young pass by gradual changes into the likeness of the parent 

 animal, and until this metamorphosis is complete, are incapable of 

 generation. Steenstrup has given the name of " nurse" to those 

 young, which, whilst departing from the parent type, remain, and 

 propagate under their own form. 



It thus happens that in the alternation of generations (to use 

 the words of Steenstrup), the parent animal produces " nurses," 

 whose descendants only, take her form. A most important 

 circumstance which characterises these nurses or " Agamozooids" 1 



1 I have rendered the word " keim-korper" by " sporula," meaning thereby a free 

 germ which is capable of development without fecundation, just as is the spore of a 

 cryptogamous plant. When the sporula? are developed in a special organ I term this 

 organ (the " keimstock" of Von Siebold) the " sporularium." Any independent form 

 from which sporulae or their equivalents alone are developed (the " ammen" or " nurses" 

 of Von Siebold, Steenstrup, &c.) I term " agamozooids." See concluding note. [ED.] 



